Let me first explain how trial by jury works in the United Kingdom, before presenting the main contentious arguments for and against the use of a jury.
The idea of a jury stems from early constitutional writings, including article 39 Magna Carta, which states that one’s rights should be impeded only on “lawful judgement of equals”. A jury today is a random selection of 12 peers, who represent a cross section of society and decide on the guilt (or lack thereof) of a peer based on fact. Mainly used in Crown Court for criminal cases (although sometimes for certain civil cases or coroner’s inquests), a jury decides the facts of a case based on the evidence presented. Although a judge will direct a jury as to the points of law and sum up the arguments and evidence given by the defence and prosecution, the jury must be truly independent in their decision regarding the facts of a case. This need for independence was shown firstly in Bushell’s Case 89 ER 2, (1670) Vaugh 135 where an appeal was allowed due to a judge’s interference in the free decision making of the jury. A verdict must usually be unanimous, although a majority vote of 10 or more may be accepted.
In summary, a jury in the United Kingdom is a (usually) 12 strong panel of laymen who decide the fate of an accused man or woman in court. It can be argued that this separation of power from the judge curtails any potential dictatorship of the state). Juries are chosen at random in accordance with the Juries Act 1974. People may be disqualified for reasons such as imprisonment or whilst on bail (Criminal Justice and Public Order Act 1988 refers).
I would argue that the most contentious issue when weighing up the fairness and effectiveness of a trial by jury is the privacy of deliberation. In allowing the jurors to confer in private and without recourse allows a freedom from pressure. There is no requirement for a reason to be given for a verdict. This can be both an advantage and disadvantage. On one hand, it can create jury equity, allowing a freedom of conscience by enabling a jury to stray from the letter of the law in order to deploy what they feel is a morally just verdict. This can be seen in R v Ponting  Crim. L.R. 318 where a jury went behind a law to acquit a man whose alleged crime, they probably felt, was in the public interest and may have saved many lives. This case led to the law in question being altered. Unfortunately, a jury may not always come to the most substantively just of decisions, as seen in R v Randle and Pottle  C.O.D 369, when the jury acquitted both Randle and Pottle despite there being no defence. The power of juries to make decisions based on conscience was strongly demonstrated in R v Wilson et al  where a jury in Liverpool acquitted four women who caused over £1,500,000 of damage to an aircraft because they felt it was in defence of an even greater crime which would have ended in many deaths.
There are, however, two instances in which a court may make inquiry into the secret deliberations of a jury. The first is when there comes to light a repudiation of the oath made by the jurors when they are sworn in. An example of this power being used by the court in this instance is R v Young  CA, when a note was sent to a defendant’s solicitor upon his conviction by a member of the jury, claiming that, overnight, an Ouija board was used overnight in a hotel by four members of the jury to decide the accused’s fate. It was decided that the privileged conjecture afforded in the juror’s room would not be extended to the overnight stay in the hotel. A retrial was ordered, and the defendant was subsequently found not guilty. Unfortunately, there may be many more instances of such foul play that will not come to light. Secondly, where material extraneous to that provided in the course of trial finds its way into the jury room, a court can make inquiry in similar vain to the aforementioned. An example of the courts power being used due to the introduction of extraneous material can be found in the case of R v Karakaya  EWCA Crim 346 where it was found that an internet search had been done on the case, which could have influenced the verdict.
The use of the internet by jurors brings to bear another issue facing justice when utilising a trial by jury: that of influence. Influence can come from many angles although the ones of most concern are those coming from the media, jury tampering, and pre-existing personal prejudice. In an era of media hype, it is very easy for the strong bias of media to influence the beliefs of the masses. This could cause the role of an impartial jury to be compromised, and tie in with the earlier issues raised with extraneous material to the case. Unfortunately, one hundred percent isolation of the jury is not always possible, and it has been the case before where the Crown Prosecution Service (CPS) has directed the media to be careful on the reporting of the case, lest it interferes with proceedings. Jury tampering- the interfering of a jury by a third party, can sometimes be an issue, however, s24 Criminal Justice Act 2003 provides the prosecution with the option of a trial without a jury in extreme circumstances. Personal prejudice is a worry for some, especially when it comes to the trial of a person belonging to an ethnic minority. There have been cases such as Sander v United Kingdom  Crim L.R. 767, where racist jokes were found to have been made amongst jurors prior to a verdict being given. This was seen to violate Article 6 of the European Convention on Human Rights, which deals with the right to a fair trial. Thankfully, research has shown that there is very little difference shown by juries regarding race (Cheryl Thomas, 2010).
In conclusion, although arguments can be made against the use of a jury in trials, I feel that the benefits of common sense amongst peers and a freedom of deliberation and conscience outweigh the drawbacks of potential interference. It is possible that judges, being part of the judiciary system, may be influenced by the state regardless of the theoretical separation of powers, and also by their own prior knowledge and opinions on the law. Trial by jury has stood the test of time for a reason: substantive justice.